WASHINGTON (Reuters) – U.S. President Donald Trump will test his ability to persuade his staunchly anti-establishment political base to get behind Republican incumbents when he wades into a bitter U.S. Senate fight in Alabama on Friday.

Trump is expected to campaign in Huntsville, Alabama, for Senator Luther Strange, who was appointed to his seat after Jeff Sessions was named Trump’s Attorney General.

Strange is trying to ward off a challenge from Roy Moore, a conservative former state Supreme Court justice, in a runoff election next week.

Trump’s involvement in the Alabama race could help bolster his strained relationship with Senate Majority Leader Mitch McConnell, whose help the president needs to advance his agenda on taxes, healthcare and immigration.

McConnell has strongly supported Strange, viewing him as a reliable vote to further the party’s legislative agenda.

A win by Moore in Alabama could embolden other insurgent candidates to challenge Republican incumbents in next year’s congressional elections.

Republican leaders fear that candidates who are too far to the right could lose to Democrats, who are seeking to wrest control of the House and the Senate in the 2018 midterm elections.

Strange, 64 and called “Big Luther” because he is 6-foot-9, has been backed by nearly $9 million of advertising from a McConnell-allied political action committee.

Moore, 70, is a religious conservative who twice lost his position as the state’s top judge. He was ousted in 2003 after refusing to comply with a federal court order to remove a Ten Commandments monument from the state Supreme Court building. Moore is also known for his opposition to gay rights.

He is popular with many of the same conservative voters who backed Trump last November.

“A lot of people love Trump and love Roy Moore,” said Ford O’Connell, a Republican strategist who has worked for Strange in Alabama.

Trump’s embrace of Strange has put him at odds with his former adviser, Steve Bannon, and the nationalistic wing of the party.

Breitbart, the conservative news site that Bannon oversees, has repeatedly attacked Strange as a Washington insider even though he has only been in the Senate for eight months, while praising Moore as an insurgent in the mold of Trump when he was a presidential candidate.

A Moore win could also cement Bannon’s status as a new political kingmaker and perhaps push him further into open conflict with Trump if the president continues to support Republican incumbents.

A political advocacy group now aligned with Bannon, Great America Alliance, recently financed a pro-Moore bus tour across Alabama, and could serve as another weapon going forward in Bannon’s ongoing war against the Republican establishment.